Ultrasonic transducers are used to generate and detect acoustic waves in solid and fluid media. They are found in a wide variety of applications, including chemical sensing, signal processing, nondestructive evaluation, and medical imaging. Based on the desired application, they can be tailored to produce and detect particular frequencies in a suitable medium. Chemical sensors, for example, excite ultrasonic waves in a solid substrate. The waves are generated in one region of the substrate, and pass through an area subject to chemical adsorption. The resultant density change in the adsorbing region alters the wave's propagation speed in the material, and this change is then detected at the other end of the sensor.
Conventional ultrasonic transducers are used to excite and detect Rayleigh (surface), Lamb (plate), and bulk waves in solid media. Transducer design and operating conditions are chosen to select a particular mode required by the application. For generation of waves in solids, three types of transducers are commonly found: piezoelectric, electromagnetic, and thermal. Each type has some drawbacks, either in efficiency, bandwidth, or ease of manufacture. Piezoelectric transducers, the most common type, require the deposition of piezoelectric material on a substrate before a voltage can be applied to the material to generate the waves. Creating the circuitry and depositing the piezoelectric material require independent manufacturing processes, making the overall process costly and inefficient. Electromagnetic transducers combine magnetic field application with fringing eddy currents to generate a sufficient force to excite ultrasonic waves. Thermal excitation transducers use lasers operating in the ablation regime to generate the waves. These transducers are clearly more complex and expensive to operate than piezoelectric transducers.
For applications requiring generation and detection of waves in air or other fluids, capacitive ultrasonic transducers are much more efficient than the above types. Examples of capacitive transducers are described in U.S. Pat. No. 5,287,331, issued to Schindel et al.; U.S. Pat. No. 5,619,476, issued to Haller et al.; and U.S. Pat. No. 5,894,452, issued to Ladabaum et al. In general, capacitive transducers contain a substrate and an array of membranes, usually circular, supported above the substrate by a dielectric material. Both the membranes and substrate have a conductive region, and can be thought of as plates of a capacitor. When an electrical signal is applied across the plates, an electric field between the substrate and membrane causes the membrane to vibrate, generating acoustic waves in the air at the frequency of the applied signal. Similarly, when an airborne acoustic wave is received by the membrane, it generates a corresponding electrical signal. If the substrate is silicon, the entire transducer can be produced using well-known silicon processing techniques, allowing for micron-scale accuracy. Capacitive transducers are particularly attractive because they can be produced inexpensively and are easily integrated with the required electronics.
Capacitive transducers are not very efficient in transferring energy from the membrane to the surrounding fluid. In fact, in some cases up to 90% of the energy is instead transferred to the support structure. However, because they are designed for excitation and detection of fluid waves, existing transducers cannot be used to excite acoustic modes of the solid support structure efficiently, and the energy coupled into the solid is wasted.
There is a need, therefore, for a capacitive ultrasonic transducer that can selectively excite and detect Rayleigh, Lamb, or bulk modes in solids.